1. Field of the Invention
This invention relates to an apparatus for detecting the flow speed of a high temperature fluid, which may be used to make a survey for determining the strata, from which petroleum, gas, hot water or steam is produced from a petroleum well or a geothermal well and also determining the estimated amount of production.
2. Description of the Prior Art
A prior art flow speed detection apparatus used for the survey noted above in a well, has a construction as shown in FIG. 14. It comprises a rotary shaft 2 having a helical rotary blade 1 secured thereto. The shaft 2 is rotatable supported by bearings 8 and 9 in a fluid passage 4 defined in a casing 3 and extends in the longitudinal direction of the passage 4. A rotor 10 is secured to the shaft 2, and a switch control permanent magnet 11 is secured to the rotor 10 at an eccentric portion. A switch holder 12 is accommodated in and secured to the casing 3 and carries flow speed detection lead switches 13 which are controlled by the permanent magnet 11.
With the above prior art flow speed detection apparatus, when the permanent magnet 11 and lead switches 13 are heated by a high temperature fluid to a temperature in excess of 260.degree. C., for instance, the magnetization of the permanent magnet 11 is reduced, and also the characteristics of the lead switches 13 are deteriorated. Consequently, the operation of the lead switches 13 for flow speed detection becomes instable, so that it is difficult to make an accurate measurement of the flow speed of the high temperature fluid (e.g., steam at 260.degree. C. or above) in a well.